In the present invention, the terahertz wave means an electromagnetic wave with a frequency in a region of 0.1 to 10 THz (1 THz=1012 Hz), accordingly, a wavelength of 0.03 to 3 mm.
Terahertz wave are expected to be applied to a wide variety of fields, such as security, information and communications, medical treatment, non-destructive inspection, molecular structure analysis, and radio astronomy. Since the terahertz wave can be transmitted through a material through which visible light cannot pass, it is possible to detect a material-specific spectrum depending on the material. For this reason, it is expected that imaging techniques or spectroscopic techniques using a terahertz wave can be very useful techniques in the field of a biopsy for a human body and the like or in the field of material inspection, such as identification of chemical substances or damage detection.
However, the wavelength of the terahertz wave is 0.03 to 3 mm, which is very long compared with the wavelength of 0.00036 to 0.00083 mm (=360 to 830 nm) of visible light. Since the spatial resolution in imaging using electromagnetic waves is limited to half the wavelength due to the diffraction limit, there is a problem in that the resolution is low compared with an image based on visible light or the like.
On the other hand, an element has been developed in which an electromagnetic wave is emitted and transmitted through a fine aperture, which is less than or equal to the wavelength of the electromagnetic wave, to obtain a fine beam spot having the same size as the aperture (refer to PTL 1 and NPL 1). By using this element, a spatial resolution comparable to the size of the beam spot can be realized. Accordingly, this element is very useful for high-resolution imaging or high-resolution spectroscopy.
However, as the size of the aperture is decreased in order to obtain a finer beam spot, the transmittance of the electromagnetic wave is dramatically reduced in inverse proportion to the fourth power of the radius of the aperture. For this reason, in order to take advantage of the finer beam spot, it becomes an issue to increase the transmittance.
PTL 2 and NPL 2 disclose an optical element in which a circular aperture is provided in the center of a metal plate and a plurality of ring-shaped grooves, which have the same center as the circular aperture and whose radii are different by a fixed length, are formed around the aperture, and have reported that the transmittance of light is increased compared with a case where there is no ring-shaped groove. Their study provides important information that the central aperture is a circle but the transmittance is increased by forming a plurality of ring grooves. When d/λ, which is the ratio of the central circular hole diameter d and the wavelength λ of the incident electromagnetic wave, is 0.25, the transmittance of 0.001 (=0.1%) is achieved.
In addition, NPL 1 discloses that the transmittance is increased by making the aperture shape of a central portion as a bow-tie shape instead of a circular shape and performing ring-shaped groove machining around the aperture, and clearly shows that the aperture shape of the central portion has a large influence on the transmittance. The transmittance is improved 5 times compared with a case where the aperture shape of the central portion is a mere circular aperture.
The inventors invented an array terahertz-wave filter, in which a number of fine apertures having a special shape are periodically placed at fixed distances, so far (refer to NPL 3). With this terahertz-wave filter, the inventors have succeeded in selectively intensifying only a specific frequency component and making the specific frequency component transmitted through the terahertz-wave filter depending on the shape of the aperture and the periodicity of the aperture arrangement.